This invention relates in general to a process for purifying silicon source materials and more specifically to a process for removing impurities from trichlorosilane or dichlorosilane or from mixtures of trichlorosilane and silicon tetrachloride and/or dichlorosilane.
Large quantities of polycrystalline silicon are produced by the hydrogen reduction of a silicon bearing reactant gas. For example, in a commonly used process trichlorosilane or mixtures of trichlorosilane with other silicon bearing reactants such as silicon tetrachloride and dichlorosilane are reduced to deposit polycrystalline silicon on a heated filament. In the semiconductor industry it is required that the polycrystalline silicon be of extremely high purity. This in turn requires that the silicon source gas be of high purity and be free from trace impurities.
Most impurities can be readily removed from trichlorosilane, dichlorosilane or silicon tetrachloride by conventional distillation techniques. Trace impurities of many primary dopants, however, are not easily removed by distillation. The primary dopants, the elements of group III and group V, include, for example, boron, phosphorus, arsenic and aluminum. All of these dopants are present in fairly high concentrations in the starting metallurgical grade silicon from which the silicon source material is produced. Additionally, both boron and aluminum are present in the quartz of the bell jar in which the polycrystalline deposition takes place and can be leached from the quartz and enter the reactant gas stream. Still further, dopant impurities including phosphorus, boron and aluminum are found in and can be leached from the metal pipes and seals of the deposition reactor. The impurities are usually present in the form of chlorides or hydrides of the dopant element such as BCl.sub.3, B.sub.2 H.sub.6, PCl.sub.3, PH.sub.3, AsCl.sub.3, AsH.sub.3 and AlCl.sub.3 or as intermediate compounds containing both H and Cl. Complexed chloride may also be present, for example chlorides containing both boron and aluminum. To reduce such contaminants to an acceptably low level by distillation would require several sequential distillation steps. It is desirable to have a purification process which achieves a still higher level of purification, unobtainable by distillation alone, which does not require the time and expense of multiple distillations.
Other processes have been proposed for purifying trichlorosilane. In one process, for example, BCl.sub.3 present as an impurity in HSiCl.sub.3 is complexed by a hydrolysis process in which water vapor is passed over the surface of the trichlorosilane. The purified HSiCl.sub.3 is then separated from the partially hydrolyzed polysiloxane residue by distillation. When this type of purification is carried out in an operating system, however, the system can be quickly clogged up by the copious amounts of solid polysiloxane residue which are formed.
It has also been suggested that elemental boron (or phosphorous) can be removed from HSiCl.sub.3 by the addition of iodine or bromine to the HSiCl.sub.3. The boron (or phosphorous) reacts with the iodine or bromine to form iodides or bromides which are readily separated from HSiCl.sub.3 by distillation. This purification technique, however, is not effective unless the boron (or phosphorous) is in the elemental state. Thus in most applications this technique is ineffective for the removal of boron and phosphorous compounds.
Because of the shortcomings of the prior art processes it has become necessary to develope a novel and improved purification system which will provide the high purity silicon source reactants necessary in the semiconductor industry.
It is therefore an object of this invention to provide an improved process for purifying silicon source materials.
It is another object of this invention to provide an improved process for purifying trichlorosilane.
It is another object of this invention to provide an improved process for purifying dichlorosilane.
It is a further object of this invention to provide an improved process for purifying trichlorosilane admixed with other silicon source gases in the recovery loop of a polycrystalline silicon production process.
It is a still further object of this invention to provide an improved process for purifying mixtures of trichlorosilane and silicon tetrachloride and/or dichlorosilane.
It is another object of this invention to provide an improved process for removing boron and phosphorous impurities from silicon source gases.